The problem of recovery of fluorine from mineral or other wastes of metallurgical processes, e.g. electrolytic refining of aluminum, is one which arises because of economic considerations and environmental or social considerations and hence has been the subject of considerable investigation. Fluorine-containing compounds are present in waste materials from such furnaces and these materials can include walls, linings and the like. Such materials also arise, as described below, from the removal of fluorine-containing compounds from waste gases of metallurgical and other processes utilizing absorption and adsorption techniques.
It is known that fluorine-containing compounds release hydrogen fluoride in the presence of water vapor at elevated temperature (pyrohydrolysis) and that the released hydrogen fluoride can be recovered in a higher concentration by a condensing or scrubbing step.
The pyrohydrolytic release of hydrogen fluoride has become significant particularly in the processing of waste materials which become available at various stages in the electrolytic production of aluminum. For instance, in the production of aluminum by fused-salt electrolysis, in which cryolite or similar fluorine-containing fluxes are usually employed, fluorine-containing constituents enter the lining of the electrolytic cell. This lining must be renewed from time to time and the old lining material which has been broken out is then available and may contain 10 to 15% by weight of fluorine, depending on the mode of operation of the cell and on the time for which the lining has been in operation.
Another fluorine-containing material will be obtained when hydrogen fluoride is removed from exhaust gases from fused-salt electrolysis by dry scrubbing. Where alumina is used as a sorbent, a chemisorption agent will become available, which is laden with hydrogen fluoride in dependence on the scrubbing conditions and must be processed because it cannot be fed to the fused-salt electrolysis as it contains other impurities formerly contained in the exhaust gases, such as carbon, sulfur, iron, silicon, phosphorus and/or vanadium.
It has been found that such waste materials can be processed by pyrohydrolysis (Opened German Specification Nos. 2,346,537 and 2,403,282), which may be combined with the recovery of additional valuable substances, such as aluminum or alkali metal (U.S. Pat. No. 4,113,832). In the last-mentioned process, the pyrohydrolytic treatment is carried out within a temperature range of about 1110.degree. to 1350.degree. C., e.g. in an expanded fluidized bed, in the presence of adequate quantities of water vapor. Alkali fluoride and hydrogen fluoride are removed from the exhaust gas. The solid residue from the pyrohydrolytic treatment is leached with an alkaline solution, and hydrated alumina is formed. Before the alkali fluoride and the hydrogen fluoride are removed from the gas, the latter is cooled by being sprayed with water or by being mixed with cold gas or by an indirect cooling.
A disadvantage of this process is that the sensible heat of the exhaust gas is wasted and the gas rate is considerably increased when the exhaust gas is cooled by a spraying of water. Similar remarks apply to the cooling by an admixing of cold gas, which involves particularly an undesired dilution of the gas, which apart from this contains only a small percentage of hydrogen fluoride. Whereas these disadvantages can be avoided by indirect cooling, this can be controlled only with difficulty because problems of corrosion and erosion arise and because the deposition of dust on the cooling surfaces decreases the coefficient of heat transfer so that a high structural expenditure is involved (for cleaning) and/or large exchange surfaces are required.